Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image forming processes below. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may include a fixing rotary body heated by a heater and a pressing rotary body pressed against the fixing rotary body to form a fixing nip therebetween. As a recording medium bearing an unfixed toner image is conveyed through the fixing nip, the fixing rotary body heated by the heater contacts an image side of the recording medium that bears the unfixed toner image and the pressing rotary body contacts a non-image side of the recording medium that does not bear the unfixed toner image. That is, the fixing rotary body heats the unfixed toner image while the pressing rotary body presses the recording medium against the fixing rotary body, thus melting and fixing the toner image on the recording medium.
Generally, a roller and a flexible endless belt are used as the fixing rotary body and the pressing rotary body with one of four examples of the combination of the roller and the belt described below.
The first example is the combination of a fixing belt as the fixing rotary body and a pressing roller as the pressing rotary body. The fixing belt is stretched over and supported by a fixing roller and a heating roller inside which a heater is disposed. The heater heats the heating roller which in turn heats the fixing belt. The pressing roller is pressed against the fixing roller via the fixing belt to form a fixing nip between the pressing roller and the fixing belt. As a recording medium bearing an unfixed toner image is conveyed through the fixing nip, the fixing belt and the pressing roller apply heat and pressure to the recording medium, thus fixing the toner image on the recording medium.
The second example is the combination of a fixing belt as the fixing rotary body and a pressing roller as the pressing rotary body. Unlike the first example described above, the fixing belt of the second example is a belt-shaped film with no roller disposed inside it. Specifically, the pressing roller is pressed against a heater disposed inside the belt-shaped film via the belt-shaped film to form a fixing nip between the pressing roller and the belt-shaped film. As a recording medium bearing an unfixed toner image is conveyed through the fixing nip, the belt-shaped film heated by the heater and the pressing roller apply heat and pressure to the recording medium, thus fixing the toner image on the recording medium.
The third example is the combination of a fixing roller as the fixing rotary body and a pressing belt as the pressing rotary body. As contrasted to the first example described above, the pressing belt, instead of the fixing belt, is stretched over and supported by a plurality of rollers disposed inside a loop formed by the pressing belt. Specifically, a stationary pressing pad disposed inside the loop formed by the pressing belt is pressed against the fixing roller via the pressing belt to form a fixing nip between the pressing belt and the fixing roller. As a recording medium bearing an unfixed toner image is conveyed through the fixing nip, the fixing roller heated by a heater disposed inside it and the pressing belt apply heat and pressure to the recording medium, thus fixing the toner image on the recording medium.
The fourth example is the combination of a fixing roller as the fixing rotary body and a pressing belt as the pressing rotary body. Unlike the pressing belt of the third example described above, the pressing belt of the fourth example is supported by a tubular guide disposed inside a loop formed by the pressing belt, not by the plurality of rollers. Like the third example, the stationary pressing pad disposed inside the loop formed by the pressing belt is pressed against the fixing roller via the pressing belt to form a fixing nip between the pressing belt and the fixing roller. As a recording medium bearing an unfixed toner image is conveyed through the fixing nip, the fixing roller heated by a heater disposed inside it and the pressing belt apply heat and pressure to the recording medium, thus fixing the toner image on the recording medium.
However, the first to fourth examples described above have a drawback of increasing the heat capacity of the fixing rotary body and the pressing rotary body. Specifically, the fixing belt of the first example stretched over the plurality of rollers has an increased loop diameter that increases the heat capacity of the fixing belt. The pressing roller of the second example pressed against the heater disposed inside the belt-shaped film has a thick rubber layer that increases the heat capacity of the pressing roller. The fixing roller of the third example pressing against the pressing pad to form the fixing nip is constructed of a plurality of layers including a thick rubber layer that increases the heat capacity of the fixing roller. Additionally, the pressing belt of the third example stretched over the plurality of rollers has an increased loop diameter that increases the heat capacity of the pressing belt. The fixing roller of the fourth example pressing against the pressing pad to form the fixing nip is constructed of a plurality of layers including a thick elastic layer that increases the heat capacity of the fixing roller.
As a result, the increased heat capacity of the fixing rotary body and the pressing rotary body may increase power consumption and lengthen a warm-up time required to warm up the fixing device and a first print time required to complete the image forming processes described above.